Manufacturing of Conductive Filaments for 3D/4D Printing through In Situ Polymerization of Biobased Thermoplastic Polyurethane/Multiwall Carbon Nanotube Composites

Flexible and stretchable conductive filaments are essential in fused deposition modeling in 3D printing, particularly for body sensing applications, as they are directly printable into any desired shape. Such filaments must meet several key criteria: they should be environmentally friendly, soft, highly conductive, and thermally stable during high-temperature 3D/4D printing. In this study, we fabricated biobased thermoplastic polyurethane (TPU)/multiwall carbon nanotube (MWCNT) composites via in situ polymerization, an eco-friendly process that does not require solvents. Furthermore, we used biobased polyol and chain extenders to produce a complex containing ≥62 wt % natural materials. These composites with rheology-controlled properties exhibited exceptional fluidity during the 3D printing process and maintained their shapes under atmospheric conditions. Additionally, we examined their electrical properties by varying the MWCNT content from 2.75 to 3.75 wt %, obtaining the highest conductivity (1.26 × 10–2 S/cm) with 3.75 wt % MWCNTs in TPU (i.e., TPU/MWCNT 3.50). A 3D printed pressure sensor fabricated using TPU/MWCNT 3.50 exhibited a stable piezoresistive sensing ability under 20% repeated compression for 6000 s. Moreover, the TPU/MWCNT 3.25 composite exhibited a shape-memory property at body temperature (i.e., 37 °C). This research promotes the technological development of 4D printing and materials for future multifunctional applications.